X-ray fluorescence (XRF) analyzers are well-known in the art, for determining the elemental composition of a sample. XRF analyzers generally include an X-ray source, which irradiates the sample, and an X-ray detector, for detecting the X-ray fluorescence emitted by the sample in response to the irradiation. Each element in the sample emits X-ray fluorescence in energy bands that are characteristic of the element. The detected X-ray fluorescence is analyzed to find the energies or, equivalently, the wavelengths of the detected photons, and the qualitative and/or quantitative composition of the sample is determined based on this analysis.
In order to produce strong fluorescence signals and detect a broad range of elements in a sample, XRF analyzers generally use an X-ray tube to irradiate the sample at a high X-ray flux. Such X-ray tubes generate a broad Brehmsstrahlung radiation spectrum, as is known in the art. The Brehmsstrahlung radiation excites the elements in the sample indiscriminately and may also scatter from the sample, creating a relatively high radiation background at the detector. In consequence, weak X-ray fluorescence signals emitted by trace elements of interest may be difficult or impossible to detect.
To overcome this problem, some XRF analyzers use secondary target excitation of the sample. The X-ray tube irradiates a secondary target of known, substantially pure elemental composition, typically a slug of a pure metal. The secondary target emits narrow-band, quasi-monochromatic radiation toward the sample at the known, characteristic X-ray emission lines of the secondary target. X-ray optics prevent the radiation from the X-ray tube from reaching the sample directly. The secondary target is typically chosen so that its characteristic emission lines coincide with one or more absorption bands of a trace element of interest in the sample. The Brehmsstrahlung background is thus substantially suppressed, and the strength of the X-ray fluorescence signal due to the trace element, relative to the background, is enhanced. Therefore, secondary target systems are generally capable of detecting trace elements at much lower concentration than ordinary, direct-excitation XRF analyzers.
Although secondary target excitation is advantageous in detection of specific trace elements, direct excitation of the sample by the X-ray tube is still faster and more convenient for analysis of one or multiple elements in moderate to high concentrations. Therefore, most laboratories will not purchase an XRF analyzer that offers only secondary target excitation, but rather require that the analyzer be capable of alternating between direct and secondary target excitation. Several such analyzers are commercially available.
FIG. 1, for example, is a schematic representation of an XRF detection system 20, similar to one that is used in the EDX 771 spectrometer, manufactured by Kevex of Valencia, Calif. System 20 comprises an X-ray tube 22 that can be shifted as indicated by an arrow 24 between a direct excitation position 26, in which tube 22 is shown by a solid line, and a secondary excitation position 28, in which the tube is shown by a dashed line. Radiation from tube 22 is directed via a selected one of a plurality of filters 30 and secondary targets 32, which are mounted on a filter and target wheel 34, toward a sample 36. X-ray fluorescence emitted by the sample is detected by a semiconductor detector 38, typically a Si(Li) (lithium-drifted silicon) detector, as is known in the art.
When tube 22 is in direct position 26, radiation from the tube passes directly through filter 30 and impinges on sample 36. Filter 30 comprises copper, for example, as is known in the art. In secondary position 28, however, tube 22 is aimed at secondary target 32, which comprises, for example, molybdenum. Radiation emitted by target 32 then impinges on sample 36. An X-ray baffle, not shown in the figure, prevents X-rays from tube 22 from striking sample 36 directly. Wheel 34 may be rotated to select among a plurality of different filters 30 and secondary targets 32.
Although system 20 allows both direct and secondary excitation of sample 36 using only a single X-ray tube 22, the movement of the tube introduces instability and, consequently, reduces the system's precision. Moreover, because of the mechanical constraints imposed by the need to move tube 22, the tube must be placed relatively far from filters 30, secondary targets 32 and sample 36. As a result, the detection efficiency of system 20, i.e., the strength of the signal received by detector 38 from a given sample relative to the power applied to drive tube 22, is comparatively low.
FIG. 2 is a schematic illustration of another XRF detection system 40, similar to one that is used in the EX-6500 XRF analyzer, manufactured by Jordan Valley Applied Radiation, of Migdal Haemek, Israel. System 40 is similar in operation to system 20, shown in FIG. 1, except that system 40 includes two X-ray tubes 42 and 44 instead of shiftable tube 22 in system 20. For direct excitation of sample 36, tube 42 is activated, whereas for secondary target excitation, tube 44 is used. System 40 thus overcomes the problem of instability described above with regard to system 20, but the need to use two X-ray tubes 42 and 44 increases the cost of the system. The mechanical constraints imposed on system 40 by the use of the two tubes 42 and 44 also lead to reduced detection efficiency.